1. Field of the Invention
The present invention relates to a dielectric waveguide line for transmitting a high-frequency signal of the microwave band or the millimeter band, and particularly to a dielectric waveguide line having a bent or branched portion.
2. Description of the Related Art
In a high-frequency circuit which handles a high-frequency signal of the microwave band or the millimeter band, a transmission line for transmitting the high-frequency signal is requested to have a reduced size and a small transmission loss. If such a transmission line can be formed on or in a substrate which constitutes a circuit, it is advantageous to miniaturization. In the prior art, therefore, a strip line, a microstrip line, a coplanar line, or a dielectric waveguide line is used as such a transmission line.
Among these lines, a strip line, a microstrip line, and a coplanar line have a structure which consists of a dielectric substrate, a signal line composed of a conductor layer, and a ground conductor layer, and in which an electromagnetic wave of a high-frequency signal propagates through the space and the dielectric around the signal line and the ground conductor layer. These lines have no problem in transmitting signals within a band of not more than 30 GHz. For transmission of signals of 30 GHz or more, however, a transmission loss is easily produced.
By contrast, a waveguide line is advantageous because the transmission loss is small also in the millimeter band of not less than 30 GHz. In order to utilize excellent transmission characteristics of such a waveguide, also a line which can tie formed in a multilayer substrate has been proposed.
In Japanese Unexamined Patent Publication JP-A 6-53711 (1994), for example, a waveguide line is proposed in which a dielectric substrate is sandwiched between a pair of conductor layers and side walls are formed by two rows of via holes through which the conduct layers are connected to each other. In the waveguide line, the four sides of a dielectric material are surrounded by pseudo conductor walls configured by the conductor layers and the via holes. whereby the region in the conductor walls is formed as a line for signal transmission. The waveguide line has a very simple structure and an apparatus can be miniaturized as a whole.
When a high-frequency circuit is to be configured, usually, formation of a bent or branched portion in a wiring circuit of a transmission line is inevitable. Particularly, In the case where a feeder line for array antennas or the like is to be formed, a branch must be formed in a wiring circuit of a transmission line.
However, a strip line, a microstrip line, and a coplanar line have a problem in that, because a signal line is not completely covered with a ground conductor layer, formation of a branch at a midpoint of a transmission line causes an electromagnetic wave to be radiated from the branch, thereby increasing the transmission loss.
As a dielectric waveguide line, furthermore, known is an NRD guide having a structure in which a dielectric line is sandwiched between two ground conductor plates and the portion between the ground conductor plates and other than the dielectric waveguide line is filled with the air. In order to form a branch in the structure, a method in which two bent lines are coupled together to form a directional coupler is employed. When a bent portion exists in a line, however, there arises another problem in that different propagation modes are produced depending on the shape and the transmission loss is increased and hence strict restriction is imposed on the design. A dielectric waveguide line is usually made of fluororesin or the like. Particularly, a line which is to be used in a high frequency region has a reduced size and hence it is difficult to work a bent portion and the like, thereby causing a further problem in that it is difficult to obtain such a line by mass production. Moreover, there is a further problem in that it is difficult to form such a line as a wiring of a high frequency circuit on or in a dielectric substrate.
A conventional waveguide has a structure in which an electromagnetic wave propagates through a space surrounded by metal walls, and hence does not produce a loss due to a dielectric. Therefore, the loss at a high frequency is small, and, even where there is a branch, a radiation loss is not produced. However, such a waveguide has a problem in that the size of the waveguide is larger than that of a transmission line using a dielectric. By contrast, a dielectric waveguide line which is filled with a dielectric of a specific dielectric constant of .epsilon..sub.r can be produced at a size which is 1/.sqroot..epsilon..sub.r of that of a conventional one. However, such a waveguide also has a problem in that it is difficult to form such a waveguide on or in a dielectric substrate.
In a dielectric waveguide line such as that proposed in Japanese Unexamined Patent Publication JP-A 6-53711 (1994), when a bent or branched portion is simply formed in a line for signal transmission which is surrounded by pseudo conductor walls configured by the pair of conductor layers and the two rows of via holes, the electromagnetic field is disturbed, thereby producing a problem in that the transmission loss is increased.
In order to produce a wiring circuit of a transmission line in which a branch for forming a feeder line for an array antenna or the like in a dielectric substrate, therefore, it has been requested to develop a branch structure of a dielectric waveguide line which can be formed in a dielectric substrate, which does not radiate an electromagnetic wave, and in which the transmission loss is small.